1,1-Dichloro-2,3,3,3-tetrafluoropropene (CFO-1214ya) is a compound useful as a raw material for synthesis of 2,3,3,3-tetrafluoropropene (HFO-1234yf) which is expected in recent years as a new refrigerant to replace 1,1,1,2-tetrafluoroethane (HFC-134a) being a greenhouse effect gas. In this specification, with respect to a halogenated hydrocarbon, in brackets after the chemical name, an abbreviation of the compound is indicated, but in this specification, instead of the compound name, its abbreviation may be employed as the case requires.
As a method for producing such CFO-1214ya, a method has been known wherein 1,1-dichloro-2,2,3,3,3-pentafluoropropane (HCFC-225ca) is, as a production raw material, dehydrofluorinated in an alkali aqueous solution in the presence of a phase-transfer catalyst or in a gas phase reaction in the presence of a catalyst such as chromium, iron, copper or activated carbon (Patent Document 1).
Here, HCFC-225ca to be used as the production raw material in the above method is usually produced as a mixture of 1,3-dichloro-1,2,2,3,3-pentafluoropropane (HCFC-225cb), 2,2-dichloro-1,1,3,3,3-pentafluoropropane (HCFC-225aa) and other isomers (Patent Document 2, Non-Patent Document 1). Therefore, it has been separated from such a mixture of isomers and purified to be used as a raw material for the above production method.
However, in such a mixture of dichloropentafluoropropane isomers, the respective isomers have boiling points close to one another, whereby it is difficult to separate and purify them by a usual separation/purification technique (such as distillation), and a multi-stage distillation or the like is required to produce highly pure HCFC-225ca on an industrial scale.
When the production including preparation of the production raw material, etc. is comprehensively taken into account in such a manner, the above-mentioned conventional method for producing CFO-1214ya is hardly regarded as a simple and economical production method.
On the other hand, CFO-1214ya obtained by such a method can be converted to HFO-1234yf by reducing it with hydrogen in the presence of a catalyst.
In such a reduction reaction, an outlet gas of the reduction reactor contains, in addition to the desired product HFO-1234yf, an unreacted raw material CFO-1214ya, an intermediate product 1-chloro-2,3,3,3-tetrafluoropropene (HCFO-1224yd) and byproducts such as 1-chloro-2,3,3,3-tetrafluoropropane (HCFC-244eb), etc.
Here, the unreacted raw material CFO-1214ya and the intermediate product HCFO-1224yd can be effectively utilized by recycling them to the reduction reactor after separating them by distillation from the desired product HFO-1234yf.
However, a byproduct HCFC-244eb has a boiling point close to CFO-1214ya and HCFO-1224yd and can hardly be separated by distillation from these compounds. HCFC-244eb is returned to the reduction reactor together with CFO-1214ya and HCFO-1224yd, but since it is an inactive compound in the reduction reaction, there has been a problem that as the operation of returning it to the reduction reactor is repeated, it is concentrated in the reduction reactor thereby to lower the production efficiency of HFO-1234yf.